The long-term objective of this program is to understand the mechansims of cellular changes fundamental to neoplastic transformation. Mammalian cells in culture represent a model with which to study growth regulation. Tumor cells have been shown to produce growth factors with mitogenic activity. Moreover, recent studies have shown that the gene products of certain tumor viruses are virtually identical to the normal cellular homologs which participate in the regulation of cell proliferation. Since mammalian cells in culture usually require undefined serum for proliferation, an assessment of the biologic impact of retroviral gene products on growth control required development of serum-free culture systems. Presently, precoating of the growth surface, and "optimized" identified as critical for cell growth in serum-free medium.